The present invention relates generally to the field of flux cored arc welding (FCAW) and more particularly to the composition of the weld wires used in the FCAW process.
Flux core arc welding is a welding process which joins, or welds, metals together via an electrical arc established between a continuous metal electrode and a metal work piece. The metal electrode used in the FCAW process is usually a consumable electrode which is fed into the FCAW process as fast as it is consumed. As mentioned above, the FCAW process operates by causing an electrical potential difference to build up between the electrode and the work piece. As the electrode approaches the work piece, the electrical potential difference between the electrode and the work piece causes a current to flow in the electrode and an electric arc to develop between the tip of the electrode and the metal surface of the work piece. This current flow heats up and melts the electrode and the surface of the work piece, generating the flow of the filler metal toward the metal surface. As the filler metal and the work piece cool, a bond or weld is formed on the metal surface of the work piece.
Referring to FIG. 1, an example of a welding gun and an arrangement for FCAW is schematically shown and discussed. A consumable welding electrode 100 is fed into the welding process through a welding gun 102 having a gas nozzle 104. Electrode 100 is melted into molten metal 106 by an electrical arc 108 established between electrode 100 and a work piece 110. An externally supplied gas, such as Ar or CO2 or mixtures thereof, enters the welding process through gas nozzle 104 in welding gun 102 and shields the electrical arc 108, the tip of electrode 100 and the pool of molten metal 106 by forming a gas shield 116. The advantages of the FCAW process include a high quality weld that can be produced faster and with very little spatter and loss of alloying elements due to the gas shield and a stable electrical arc.
The consumable electrode in FIG. 1, which is melted by the electrical arc, is transported by the arc to the work piece to serve as a surfacing or cladding material. The arc produces the heat for the welding process and is maintained by the electron flow between a cathode (positive terminal) and an anode (negative terminal). In the FCAW process both the consumable electrode and the work piece can function as a cathode or an anode.
One way to stabilize the arc in the FCAW process is by altering the composition of the welding wire electrode to add fluxing and alloying elements. Wires for FCAW are flux-cored wire electrodes comprising a flux filler core encapsulated by a metal sheath. The core of the wire electrode is made of fluxing and alloying compounds which become deposited weld materials. The composition of the core affects the composition and physical characteristics of the weld metal. Fluxing and alloying compounds contained in the core are selected to function as deoxidizers, alloying elements and arc stabilizers, and may provide additional shielding gas, such as CO2. Flux cored wires provide the ability to add various materials to the core in order to improve the welding characteristics and conditions of the FCAW process. Therefore, it would be desirable to have an electrode wire having a flux core composition affecting creation and maintenance of a stable arc in the FCAW process while exhibiting the desired high deposition and fast fill characteristics.
It is known that a submerged arc process currently used to hard surface such work pieces as steel mill rolls often generates too much heat on smaller diameter rolls. As a result, the molten metal tends to slide off the surface. The resulting bead turns out to be rough and requires labor intensive finishing to obtain a surface of suitable quality.
It is also known that an open-arc process in common use today is prone to produce excessive amounts of smoke and spatter and often results in poor quality welds that may contain porosity. It would be desirable, therefore, to find a way to hard surface smaller work pieces by improving the quality of the weld beads and reducing the post-welding finishing and machining of the resurfaced work piece.
The present invention provides a welding wire comprising a core produced from powdered materials. The flux core comprises a composition alloyed with a combination of alloying and fluxing elements comprising Cr and N. Other alloying elements important for hard surfacing can be, among others, Mo (which enhances corrosion resistance at higher temperatures), Nb (which facilitates formation of stable niobium carbides, reducing the percentage of available C in the weld deposit, therefore, reducing the likelihood of formation of chromium carbides susceptible to pitting corrosion). Fluxing compounds are present in the core composition to form slag, act as arc stabilizers, provide deoxidization and additional arc shielding during their decomposition. The flux core can constitute between 10% Wt and 50% Wt of the welding wire. The present invention also comprises a metal sheath encapsulating the flux core and constituting between 90% Wt and 50% Wt of the welding wire. The alloying elements can be added to the core or to the sheath of the wire, depending on a particular application or manufacturing process. The sheath material used to manufacture the wires can be mild steel, 300 Series stainless steel, or 400 Series stainless steel. The composition of the welding wire makes it possible to use the wire in the FCAW welding process without destabilizing the welding arc. One of the advantages of using nitrogen as an alloying element in the flux core is the fact that nitrogen-containing weld deposits resist pitting corrosion much better than the weld deposits without nitrogen. It is thought that replacing carbon with nitrogen in the flux core of the welding wire reduces the susceptibility of the resulting weld to pitting corrosion.
Additionally, the present invention contemplates a welding apparatus comprising a welding gun having a device for feeding an electrode into the welding gun. The electrode comprises a metal sheath encapsulating a flux core having a core composition, wherein the core composition is alloyed with a combination of alloying and fluxing elements comprising Cr and N, as well as others, and wherein the flux core constitutes between approximately 10% and 50% of the total weight of the wire electrode. The welding apparatus has means for shielding the electrode during the welding process by using a gas shield. The gas shield can comprise CO2, or a mixture of Ar and about 5% to 25% of CO2. According to the invention, a welding process utilizing the novel welding wire is provided. The process comprises obtaining an electrode, comprising a metal sheath encapsulating a flux core. The flux core has a composition comprising fluxing and alloying elements, such as Cr and N. Additionally, the welding process comprises using a welding apparatus with a device for feeding the electrode into the welding apparatus and a device for supplying a shielding gas into the welding apparatus. The present FCAW process uses a shielding gas to shield the nitrogen-bearing electrode from the surrounding environment during welding. The welding process comprises connecting the welding apparatus to a power source, operating the welding apparatus to generate an arc, feeding the electrode into the welding apparatus and supplying the shielding gas to the welding apparatus to shield the electrode and the arc.
One of the applications of the wire and welding process of the present invention is hard surfacing of continuous caster steel mill rolls. These are often small diameter hollow cylinders. As an example, the rolls welded by the wire and method of the present invention can have a diameter of about 4 inches to 12 inches.
Such smaller hollow body rolls are hard to weld by employing the traditional submerged arc welding typically used for larger steel mill rolls. On smaller rolls the submerged arc welding process generates excessive amounts of heat, which causes the surface of the metal work piece to stay molten too long to result in a clean weld.
The open arc, flux cored process presents it""s own set of problems, which may include excessive smoke, excessive spatter, and porosity of the deposited weld metal. Both processes often produce poorer quality beads needing further machining and finishing.
The gas-shielded FCAW process generates less heat and makes it possible for the weld beads to stay on the surface of a roll, forming a smoother surface and requiring less post-welding grinding and finishing. With the wire and method of the present invention, the hard surfacing alloying elements produce a cleaner, porosity free weld with little smoke, low spatter, and smoother beads, resulting in less after welding finishing work.
The above discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.